1. Field of the Invention
The present invention relates to an improved proximity fuze for a tube launched projectile carrying a payload.
2. Discussion of the Prior Art
The use of a fuze on a payload carrying projectile has two main functions. Firstly, it protects a handler of the projectile from inadvertent detonation or activation of the payload from when the projectile is manufactured until it is at least several meters from the site from which is was launched. Secondly, it initiates detonation or activation of the payload at a range from a target or from the ground, which range is appropriate to the function of the payload. The appropriate range may be contact with the target, after contact with the target, time from launch or proximity to the target. It is the latter type of fuze, a proximity fuze, to which the present invention relates.
Current proximity fuzes use a sensing technique known as xe2x80x9cdopplerxe2x80x9d proximity sensing. This technique requires a continuous constant wavelength radio frequency signal to be transmitted by the fuze such that when the projectile approaches its target, the fuze starts to receive its own transmitted signal reflected back from the target. This return or echo signal then interferes with the transmitted signal from the fuze in a sequence of constructive and destructive interference as the projectile gets closer to the target. For a particular reflectivity of target and given that the power of the radio signal transmitted by the fuze and the gain of the receiver which detects the return signal are set, the distance of the projectile from the target when the fuze first starts to detect its own returned transmitted signal can be predicted. Based on this prediction of the proximity of the projectile to the target when the return signal is first detected by the fuze, the fuze can be made to count through successive cycles of constructive interference. The number of cycles of constructive interference that the fuze counts before it activates the payload can thus be set to fix the proximity of the fuze to the target when the fuze activates the payload.
One problem with this type of fuze is that different targets will have different reflectivities and so the distance from the target at which the fuze first starts to receive a reflected signal will vary from target to target. Accordingly, the distance from the target at which the payload is activated, ie. after the predetermined number of cycles of constructive interference have been counted, will vary from target to target. This reduces the accuracy of such xe2x80x9cdopplerxe2x80x9d type proximity fuzes. This lack of accuracy is generally overcome in the field by firing a first projectile and observing its proximity to the target when it is activated. Then the number of cycles of constructive interference which are counted before activation can be adjusted for subsequently fired projectiles directed at the same target. However, an increasing number of potential applications require a higher accuracy fuze without the need for adjustment prior to use.
A further reduction in accuracy is caused because the frequency of the radio signal transmitted by the fuze is not generally as stable as required and so the timing of successive cycles of constructive interference will not be accurate. Further inaccuracies can be added because the ability of the fuze to detect cycles of constructive interference is dependent on background noise levels. Finally, the simple algorithm used by these xe2x80x9cdopplerxe2x80x9d type proximity fuzes and the continuous transmission at one frequency can make them relatively easy to detect and to cause the fuze to malfunction, either by accident, eg. if the ammunition travels through a radar beam or by active sensor jamming measures taken by the enemy.
The aim of the present invention is to provide an improved proximity fuze which can overcome at least some of the problems discussed above. In particular the present invention aims to provide a more accurate proximity fuze which is more difficult to defeat by active sensor jamming measures than the currently used xe2x80x9cdopplerxe2x80x9d type fuzes.
According to a first aspect of the present invention there is provided a proximity fuze for use in a tube launched projectile carrying a payload, comprising:
oscillator means for generating a radio frequency signal which has a varying frequency,
a single antenna means for transmitting the radio frequency signal and for receiving an echo of the radio frequency signal,
first signal processing means for generating a range signal corresponding to the time delay between the transmission of the radio frequency signal and the receipt of the echo signal,
second signal processing means for comparing the range signal with a reference signal and depending on the result of the comparison generating an activation signal for activating the payload,
wherein a directional coupling means is used for coupling the radio frequency signal from the oscillator means to the antenna means and to the signal processing means and for coupling the echo signal from the antenna means to the signal processing means, and wherein the second signal processing means comprises a threshold detector, a peak detector and a comparator, the threshold detector being for allowing the comparator to utilise the output from the peak detector only once the range signal has reached a predetermined magnitude.
The transmitted signal is reflected by the target for the projectile or by the ground and the antenna means receives an echo signal, ie. the component of the transmitted radio frequency signal which is reflected off the ground or the target. As the transmitted radio frequency signal has a time varying property, this property of the received signal will be different from that of the transmitted signal by an amount dependant on the rate at which the property of the radio frequency signal varies and the time it takes for the radio frequency signal to travel between the antenna to the target or the ground and back. As the time it takes for the radio frequency signal to travel between the antenna to the target or the ground and back is directly related to the distance or proximity of the antenna (and thus the projectile) from the ground or the target, the difference between the relevant property of the transmitted and received signal can be used to generate a range signal indicative of the distance between the projectile and the ground or target. This range signal can then be compared with a reference signal having the value that the range signal will take when the projectile is in the correct proximity to be activated. When the range signal and the reference signal are the same an activation signal is generated to activate the payload of the projectile, which could be, for example, high explosives, obscurance material or a lighting flare.
The fuze according to the present invention does not rely on the attenuation in amplitude of the echo signal as compared to the transmitted signal as an indicator of the distance it has travelled to reach the target (or the ground) and to return. Therefore, the ranges calculated in the fuze according to the present invention are not dependent on the reflectivity of the target (or the ground). Provided the echo signal has an amplitude sufficient to be received by the antenna means then the comparison of the time varying property of the transmitted signal and the echo signal can be used to generate an accurate indication of the range of the projectile from the target (or the ground).
In a first preferred embodiment the amplitude of the echo signal does not alter the calculated range, and the radio frequency signal is transmitted continuously. This preferred embodiment is suitable for use at relatively close proximities, such as 0.5 m to 500 m.
The signal processing means of the fuze preferably comprises mixing means for mixing a part of the transmitted signal with a part of the echo signal to generate a range signal comprising an intermediate frequency signal with a frequency corresponding to the difference between the frequency of the transmitted radio frequency signal and the received echo signal.
The frequency of the transmitted signal and the echo signal are compared by mixing them in a mixing means to generate a intermediate frequency signal with a frequency equal to the difference between the frequency of the transmitted and the frequency of the received signal. The rate at which the frequency of the transmitted signal changes is known and so the difference in the frequency of the transmitted and received signal can be used to calculate the time taken for the transmitted signal to travel to the target (or the ground) and return to the antenna means and accordingly can be used to accurately calculate the distance of the projectile from the target (or the ground). To simplify the range calculation it is preferred that the oscillator generates a radio frequency signal for transmission by the antenna means which has a linearly varying frequency.
As the transmitted frequency changes and as the frequency of the received signal required to trigger activation of the payload of the projectile is not the same as the transmitted frequency and also changes, the fuze according to the present invention is more difficult to defeat using active sensor jamming measures than a xe2x80x9cdopplerxe2x80x9d type fuze which transmits and receives a single frequency.
Preferably, the oscillator means comprises a voltage controlled oscillator and preferably the fuze additionally comprises means for generating a linear triangular waveform voltage for driving the voltage controlled oscillator.
In order to reduce the complexity and cost of the fuze a single antenna means is used for transmitting the radio frequency signal and for receiving the echo signal. This allows a directional coupling means to be used for coupling the radio frequency signal from the oscillator means to the antenna means and to the signal processing means and for coupling the echo signal from the antenna means to the signal processing means. A preferred directional coupling means which provides low cost and small size is a quarter wavelength branchline coupler comprising four ports. If such a coupler is used, to save space the oscillator means and the mixing means can be located within an area bordered by the quarter wavelength transmission lines of the branchline coupler.
According to an alternative embodiment of the present invention which is suitable for use at longer ranges, such as 500 m to 1 km the fuze is adapted to allow operation in a pulsed mode for transmitting a pulsed radio frequency signal. Here the time varying property can be amplitude as the amplitude of the radio frequency signal varies between zero and the amplitude with which the pulses are transmitted. According to this embodiment a series of pluses of radio frequency signal are transmitted, for example 10 pulses, and the echo signal pulses are received after travelling to the target (or the ground) and back. The time delay between transmitting the pulses and receiving them can be used to determine the range of the projectile from the target (or the ground). Furthermore, the difference in time delay for successive pulses can be used to determine the relative velocity between the target (or the ground) and the projectile. This embodiment is suitable for use at longer ranges because at short ranges the time delay between transmitted and received signals is too short for transmitted and received pulses to be discriminated, that is an echo of a pulse will be received while that pulse is still being transmitted.
This pulsed mode of operation can be used in combination with the frequency varying mode of operation described above. At long range the fuze can initially operate in pulsed mode to determine its proximity to the target (or the ground) and its velocity relative to the target (or the ground) and depending on the calculated distance and velocity can shut down until it is closer to the target (or the ground). Then, at a predetermined shorter range the fuze can be operated in frequency varying mode to calculate its range from the target (or the ground) and activate the payload of the projectile when the calculated range is the desired value for activation of the payload.
The second signal processing means further includes two filters of differing time constants, the outputs of both filters being connected to the input of the threshold detector.
According to a second aspect of the present invention there is provided a tube launched projectile comprising a fuze as defined above.